1. Field of the Invention
The present invention concerns combustion devices and more particularly a method and apparatus for heating fuel prior to its injection into a combustion chamber of a combustion device such as an internal combustion piston engine, to an elevated temperature level sufficient to achieve hypergolic combustion with only negligible delays in ignition and combustion.
2. Description of the Prior Art
It has heretofore been proposed to achieve so called "hypergolic" combustion, particularly of hydrocarbon fuels in an internal combustion engine, such that ignition delay and the time interval required to complete combustion are both negligible once the fuel is introduced into an oxidizing atmosphere.
For a detailed discussion, see U.S. Pat. No. 4,448,176; SAE paper No. 850089 "Hypergolic Combustion in an Internal Combustion Engine"; and, SAE paper No. 820356, "The Influence of Initial Fuel Temperature on Ignition Delay"; each of which are hereby referenced.
Hypergolic combustion has a number of advantages, as detailed in the aforementioned references, particularly in the context of internal combustion reciprocating engines.
As also detailed in the aforementioned references, ignition is believed to occur due to the need for the fuel molecules to be dissassociated into radicals in order to combine with oxygen molecules, which themselves must be disassociated for oxidation to occur. In a typical combustion process, an ignition device such as a spark plug causes a localized increased concentration of fuel radicals in a charge of fuel-air mixture, sufficient for initiation of combustion at that locale. The release of heat from that localized combustion in turn causes additional dissassociation of adjacent fuel molecules to enable combustion to propagate through the entire charge of fuel-air mixture.
As discussed in the aforementioned U.S. Pat. No. 4,448,176, if there is a pretreatment of the fuel such as to cause dissassociation of a critical proportion of molecules in each quantity of fuel, much higher than the proportion occurring at normal temperatures, there is an "activation" of the fuel. This proportion has been computed in the above cited references to be approximately 3.times.10.sup.-5 %.
If the activating pretreatment is done prior to mixing of the fuel with air, an ignition device is not needed to initiate combustion, and in fact achieves substantially instantaneous ignition and combustion of the fuel upon coming into contact with an oxidizer.
In order that such an increased, critical proportion of fuel molecules be dissassociated into radicals, energy must be expended to bring this proportion of the fuel molecules to the relatively high energy state corresponding to the dissassociated condition of the fuel molecules.
As described in the aforementioned U.S. Pat. No. 4,448,176, if fuel is heated to relatively elevated temperatures in excess of 1000.degree. F., this causes such critical proportion of fuel molecules to be dissassociated to form radicals, since such proportion of the fuel molecules is thereby brought to a high energy state.
In co-pending application Ser. No. 06/812,863 filed on Dec. 26, 1985, there is described a method and system for heating of the fuel to such elevated temperatures by a regenerative heat exchange process, in which the fuel is circulated through a vessel disposed directly in the combustion chamber, with the combustion chamber insulated to retain heat therein. This arrangement, when combined with preheating of the fuel, as with an exchanger in the engine exhaust system, is able to heat the fuel to such elevated temperatures sufficiently, that upon injection into the combustion chamber hypergolic combustion will result.
A disadvantage of regeneratively heating the typical hydrocarbon fuel is the tendency for excessive cracking of the fuel molecules at high temperatures, and the resultant formation of coke, tending to clog the fuel flow passages. It has been discovered that if the fuel is sustained at the elevated temperatures for only very short time periods, this will avoid or alleviate this problem.
Also, while such aforementioned regenerative heating method and system will efficiently heat the fuel to such elevated temperatures, a relatively complex fuel circulation system is necessitated, and the tendency for coke formation is higher due to the longer times required to achieve heating by heat exchange with the products of combustion produced in previous combustion cycles.
Accordingly, it is an object of the present invention to provide an arrangement and method for heating of fuel prior to combustion in a combustion device such as an internal combustion engine to the elevated temperature levels necessary to achieve hypergolic combustion, in which a very rapid heating of the fuel is achieved.
It is a further object of the present invention, to provide such method and arrangement in which the fuel heating system provides the high temperature heating of the fuel, but the residence time of such heated fuel in the flow passages is very short.